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Chapter 5 Periodic Classification of Element

Till now 118 elements are known with a well known classified group and period in modern periodic table. It sounds we easy to us only 118 elements are yet discovered but many more to come. During the 18th century only 30 elements were known with different physical & chemical properties and it is difficult to study. Therefore these elements are classified into round & period and later during the 19th century more elements are discovered and they are arranged according similarities and dissimilarities in much time and effort of a chemist are required to finally develop the most appropriate classification of element. This chapter is all about the brief historical development of classification of elements with their merits and demerits.

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Exercise 1

  • Q1 Did Döbereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.
    Ans:

    Yes, there is only one triad of Dobereiner’s triads exists in the columns of Newlands’ octaves. The elements Li, Na, and K of Dobereiner’s triads also occurred in the columns of Newlands’ octaves.

    Dobereiner could only find three triad from the elements known at that time.

    Dobereiner’s triads

    Li         Ca         Cl
     
    Na       Sr          Br
     
    K         Ba          I
     
    Newlands’ octaves
    H Li Be B C N O
    F Na Mg Al Si P S
    Cl K Ca Cr Ti Mn Fe
    Co Cu Zn Y In As Se
    Br Rb Sr Ce and La Zr - -

     


    Q2 What were the limitations of Döbereiner’s classification?
    Ans:

    According to Dobereiner’s classification: All elements present during that period cannot be classified into a group of three on the basis of their properties.


    Q3 What were the limitations of Newlands’ Law of Octaves?
    Ans:

    Limitations of Newlands’ law of octave are as follow:

    (i) This law is only applicable upto the calcium element. Elements after calcium don’t possess the properties similar to that of first.

    (ii) He assumed that only 56 elements exist in nature, no more element can be further discovered.

    (iii) Newland’s adjusted 2 elements in the same slot to fit the element in the table. Eg. cobalt and nickel are placed in the same slot with columns having fluorine, chlorine and bromine which have completely different properties.


Exercise 2

Exercise 3

Exercise 4